Digital print output devices, such as copiers, printers, and printing presses, often operate in a binary mode (i.e., a printer typically either deposits colorants or ink on a substrate or does not deposit colorants or ink on the substrate at a specific location). The resolution of a digital printer is typically expressed in dots per inch (“dpi”), which describes the number of locations per inch in which the printer can either print a dot, or not print a dot. For example, a 600 dpi laser printer can print (or not print) 600 dots per linear inch, whereas a 2400 dpi image setter can print (or not print) 2400 dots per linear inch.
In many instances, a user may desire to print a digital image on a particular printer, but the resolution of the digital image differs from the printer's resolution. For example, many electronic scanners and software document creation applications operate at a medium resolution of 300 dpi, whereas many conventional printers operate at 600 dpi or higher. Thus, it is often necessary to convert the resolution of image data from a first resolution to a second resolution.
One previously known technique for image resolution conversion involves interpolating between neighboring image values at a first resolution to obtain image values at a second resolution. For binary image data, in which the data only may have values of “0” or “1,” however, such techniques are difficult to implement. For example, Keithley et al. U.S. Pat. No. 6,356,670 (“Keithley”) describes methods for converting diffused black-and-white images to lower spatial resolutions. In particular, binary image data are created in a first array at a first resolution, the image data are converted to scaled image values in a second array at a second resolution by interpolating between neighboring image values in the first array, and then the scaled values are diffused to produce binary values in the second array. Keithley's technique is fairly complex, and appears to be limited to converting from a higher resolution to a lower resolution.
Other previously known techniques are limited to integer or small ratio conversion. For example, such techniques may be used to convert image data at 300 dpi to 400 or 600 dpi, but may not be used to convert the data to 720 dpi. In many instances, however, it may be useful to convert the resolution of binary image data using arbitrary conversion ratios.
In view of the foregoing, it would be desirable to provide methods and apparatus for simply converting binary image data at a first resolution to binary image data at a second resolution.
It also would be desirable to provide methods and apparatus that may be used to convert binary image data at a first resolution to binary image data at a second resolution that may be higher or lower than the first resolution.
It also would be desirable to provide methods and apparatus for converting binary image data at a first resolution to binary image data at a second resolution using arbitrary conversion ratios.